Silent phase commutation in a three-phase brushless DC motor

ABSTRACT

A method drives a three-phase motor having first, second, and third coils. The method electrically connects the first coil to a first voltage reference and the second coil to a second voltage reference while leaving the other coil floating during a first driving phase. During a second driving phase, the first coil is electrically connected to the first voltage reference and the third coil is electrically connected to the second voltage reference while the second coil is left floating. During a transition phase that immediately follows the fast driving phase and immediately precedes the second driving phase, the second coil is electrically connected alternately to the first and second voltage references. By alternately connecting the second coil to the first second voltage references and during the transition phase, the method causes the current through the second coil to reduce to zero at a slower rate than prior methods. This enables the variations of the currents that the two phases in commutation, the second and third coils, to happen in a way that maintains their sum constant, as much as possible. This reduces the torque ripple during the phase commutations and its accompanying acoustic noise.

TECHNICAL FIELD

The present invention relates to DC motors, and more particularly, toreducing the noise generated during phase commutation of a three phase,current controlled motor.

BACKGROUND OF THE INVENTION

Three-phase brushless DC motors have many uses, among which are asspindle motors for computer hard disk drivers, digital video disk (DVD)drivers, CD players, and tape-drives for video recorders. Such motorsare recognized as having the highest torque and power capability for agiven size and weight Compared to DC motors employing brushes, brushlessDC motors enjoy reduced noise generation and improved reliabilitybecause no brushes need to be replaced due to wear.

FIG. 1 shows such a three-phase brushless DC motor 10 with three phasesA, B, C having three coils 12, 14, 16 connected to each other in aY-configuration at a center tap 18. As is well-known, the coils 12, 14,16 are part of a stator that causes a permanent magnet rotor to rotate.The first coil 12 (phase A) is connected to a supply voltage Vret by afirst high-side transistor 20 and to ground via a first low-sidetransistor 22 and a sense resistor 23; the second coil 14 (phase B) isconnected to the supply voltage Vret by a second high-side transistor 24and to ground via a second low-side ransistor 26 and the sense resistor;and the third coil 16 (phase C) is connected to the supply voltage Vretby a third high-side transistor 28 and to ground by a third low-sidetransistor 30 and the sense resistor 23. Each of the transistors is anNMOS transistor as is typical. Represented in FIG. 1 by voltage supplysymbols are respective back EMF sources EA, EB, EC that are inherentlyinduced by the permanent magnets of the rotor while the rotor isrotated.

This type of motor is driven by exciting its phases in a suitablesequence while always keeping two phases under power and leaving a thirdphase in tristate or floating with a high impedance (Z). For example,assume that initially the fist high-side transistor 20 and the secondlow-side transistor 26 are activated with high control signals on theirgates while the other transistors are inactive. This results in acurrent IA through the first phase A having a value of +I, a current IBthrough the second phase having a value of −I, and zero current ICthrough the third phase as shown in FIG. 2. At predetermined instances(t1, t2, . . . ) the driving of the phase switches so that current isdriven through the phase that was previously floatin and one of theother phases is left floating such that the algebraic sum of thecurrents in the three phases are always equal to zero. In FIG. 2 thedriving sequence is as follows where the first letter indicates thephase of positive current flow and the second letter indicates the phaseof negative current flow:

AB-AC-BC-BA-CA-CB.

In the instant of commutation from one stage to another (instances t1,t2, . . . ), if the current front were infinite, one would ideally finda system without perturbations. For example, at the instant t1, thephase A would maintain the current +I while the phases B and C wouldexchange the current flow, one from −I to 0 and the other from 0 to −I.

In reality, because of the presence of different time constants in thecircuit, the commutation fronts of the two currents (IB and IC in theexample of instant t1) would be non-ideal and non-synchronous. That is,the current IC increases more slowly than the current IB decreases. Thistranslates into a variation of the current IA instead of the current IAremaining constant. The current variation generates torque ripple in themotor and much acoustic noise.

Analyzing the scheme of FIG. 1, it is possible to determine the reasonsfor the different commutation times in the two interested phases. Atinstant t1 (before the commutation) we would have:

VoutA=Vret IA=I

VoutB=0 IB=−I

VoutC=Vct IC=0

Vct=½ Vret

Given that the phases are out of phase by 120°, the electromotive forcesdriven will be instantaneously algebraically summed to zero. Thecommutation moreover happens at the instant t1 at the end of optimizingthe torque ripple in the system. The back EMF in the three phases wouldhave the following values: EA=E, EB=EC=−E/2, where E equals the maximumback EMF.

In the instant just after the commutation we would have:

VoutA=Vret

VoutB=Vret+Vbe (due to the current of coil B recirculating in theintrinsic diode of second high-side transistor 24, where Vbe equals thedrop across that intrinsic diode)

VoutC=0

Vct=⅔ Vret.

The back EMF values will remain instantaneously unchanged.

The voltage across the second coil 14 is therefore:

VoutB−(Vct+EB)=Vret+Vbe−(⅔Vret−E/2)=⅓Vret+Vbe+E/2,

while the voltage across the third coil 16 is:

VoutC−(Vct+EC=0−(⅔Vret−E/2)=E/2−⅔Vret.

The two voltages will therefore be significantly different, creatingdifferent time constants of charge/discharge of the two currents (IBwill be reduced more quickly than C will be increased).

SUMMARY OF THE INVENTION

An embodiment of the invention is directed to a method and motor driverfor driving a three-phase motor having first, second, and third coils.The method electrically connects the first coil to a first voltagereference and the second coil to a second voltage reference whileleaving the other coil floating during a fist driving phase. During asecond driving phase, the first coil is electrically connected to thefirst voltage reference and the third coil is electrically connected tothe second voltage reference while the second coil is left floating.During a transition phase that immediately follows the first drivingphase and immediately precedes the second driving phase, the second coilis electrically connected alternately to the first and second voltagereferences. By alternately connecting the second coil to the first andsecond voltage references and during the transition phase, the methodcauses the current through the second coil to reduce to zero at a slowerrate than prior art methods. This enables the variations of the currentswhat the two phases in commutation, the second and third coils, tohappen in a way that maintains their sum substantially constant. Thisreduces the torque ripple occurring during phase commutations and itsaccompanying acoustic noise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a three-phase brushless motor accordingto the prior art.

FIG. 2 is a timing diagram of the currents through the respective phasesof is the motor shown in FIG. 1.

FIG. 3 is a circuit diagram of a three-phase brushless motor accordingto an embodiment of the present intention.

FIG. 4 is a timing diagram of the control signals applied to the gatesof the control transistors of the motor shown in FIG. 3.

DETAILED DESCRIPTION

A three-phase brushless motor 50 according to be present invention isshown in FIG. 3. The motor 50 includes three coils 52, 54, 56 connectedto each other in a Y-configuration at a center tap 58. The first coil 52is connected to a supply voltage Vret by a first high-side transistor 60and to the ground via a first low-side transistor 62 and a senseresistor 64; the second coil 54 is connected to the supply voltage Vretby a second high-side transistor 66 and to ground via a second low-sidetransistor 68 and the sense resistor 64; and the third coil 56 isconnected to the supply voltage Vret by a third high-side transistor 70and to ground by a third low-side transistor 72 and the sense resistor64. Each of the transistors 60-62, 66-68, 70-72 is typically an NMOStransistor. Represented in FIG. 3 by voltage supply symbols arerespective back EMF sources EA, EB, EC that are inherently introduced bythe permanent magnets of the rotor while the rotor is rotated.

The motor 50 also includes control logic 74 coupled to the gates of thecontrol transistors 60-62, 66-68, 70-72 in order to control theactivation and deactivation of those transistors. In this matter, thecontrol logic 74 controls which of the coils 52-56 are supplied withcurrent and in which direction the current flows through those coils.The control logic 74 includes a pulse width modulation (PWM) signalgenerator 76 that generates PWM control signals as described in moredetail below. The control logic is coupled to an output of a comparator78 having an inverting input coupled to a voltage reference (Vref) and anon-inverting terminal coupled to a node 80 between the sense resistor64 and each of the low-side transistors 62, 68, 72.

Shown in FIG. 4 is a schematic timing diagram of the control signalsapplied to the gates of the control transistors 60-62, 66-68, 70-72 bythe control logic 74 of FIG. 3. During successive driving phases, wewill have one phase driven in PWM, one phase short-circuited towardground (through the sense resistor 64), and the third phase remains intristate. In a first driving phase D1, the first coil 52 is driven inPWM, the second coil 54 is coupled to ground, and the third coil 56 isin tristate. Coil 52 drive is accomplished by driving the gate of thefirst high-side transistor 60 with a first PWM control signal, drivingthe gate of the first low-side transistor 62 with a second PWM controlsignal (an inversion of the first PWM control signal). Coil 54 drive isobtained driving the second low-side transistor 68 with a constant highsignal. Remaining transistors 66,70,72 are driven by applying logic lowcontrol signals. The current IA through the first coil 52 will be I, thecurrent IB through the second coil 54 will be −I and the current ICthrough the third coil 56 will be zero. The comparator 78 enables thecontrol logic 74 to control the current through the coils by adjustingthe first and second PWM control signals as needed based on the currentIs through the sense resistor 64 as detected by the comparator.

If we want to effectuate a phase change between the phases B and C(coils 54, 56), at the end of the phase chance we should have IB=0 andIC=−I. At an instant t1, beginning a first transition phase I1 from thefirst driving phase D1 to a second driving phase D2, the control logic74 drives phase A (coil 52) with a duty cycle equal to 100%, drivesphase B (coil 54) with a PWM which is an inverted version of the oneused in driving phase D1 on phase A, and couples phase C (coil 56) toground. Specifically, during the first transition phase I1 the controllogic 74 applies a constant high control signal to the gate of the firsthigh-side transistor 60; a constant low signal to the gate of the firstlow-side transistor 62; the second and first PWM control signals to thesecond high- and low-side transistors 66, 68, respectively; a constantlow control signal to the third high-side transistor 70 and a constanthigh control signal to the third low-side transistor 72.

By applying the PWM control signals to the second high- and low-sidetransistors 66, 68, the absolute value of the current IB through thesecond coil 54 will tend to decrease while the second high-sidetransistor 66 is turned ON by the second PWM control signal and willtend to increase when the second low-side transistor 68 is turned ON bythe first PWM control signal. The time of total discharge of the secondcoil 54 will be therefore longer than it would be if the same coil 54had been immediately put in tristate as in the traditional systems. Itwill be appreciated that the application of the first and second PWMcontrol signals could be reversed during the first transition phase I1,although with a less effective reduction of the discharge rate of thesecond coil 54.

The current IC through the third coil 56 will grow with two differenttime constants according to whether the second high-side transistor 66or the second low-side transistor 68 are turned ON by its respective PWMcontrol signal during the first transition phase I1. When the secondhigh-side transistor 66 is turned ON, the voltage at the center tap 58is higher than when the low-side transistor 68 is turned ON. As aresult, the time constant of the current IC through the third coil 56 ishigher when the second high-side transistor 66 is turned ON and lowerwhen the second low-side transistor 68 is turned ON. Overall whathappens is a slowing of the discharge of the second coil 54 which willbe forced to adapt itself to the time constant of the third coil 56.

The current IA through the first coil 52 will be in each instant equalto the sum of the two currents IB and IC, Because the first high-sidetransistor 60 is kept ON during the entire first position phase I1, thecurrent IA through the first coil 52 will increase throughout the firsttransition phase. The current IA will be substantially constant in thefirst transition phase I1 because the variations in the currents IB andIC will substantially offset each other.

To determine when the first transition phase I1 is completed andtherefore when should enter the second driving phase D2, the motor 50employs the comparator 78 to measure the current Is through the senseresistor 64. During the portion of the first transition phase I1 inwhich the second high-side transistor 66 is ON, the current Is throughthe sense resistor 64 is equal to IC, while during the portion of thefirst transition phase I1 in which the second low-side transistor 68 isON, the current Is through the sense resistor 64 is equal to the sum ofthe currents IB and IC. For this reason, measuring the current Is in thesense resistor 64 when only high side transistor 66 is ON, we can decidewith security that the transition phase is terminated when Me voltageIs * Rs (resistance of sense resistor) reaches the voltage referenceVref. In response to determining that the voltage across the senseresistor 64 reaches the voltage reference Vref, the comparator 78 sendsa signal to the control logic 74 which causes the control logic to endthe first transition phase I1 and begin the second driving phase D2.

During the second driving phase D2, the first coil 52 is again driven inPWM, the second coil 54 is in tristate, and the third coil 56 is coupledto a ground through the sense resistor 64 and the third low-sidetransistor 72. As a result, the current IA through the first coil 52equals I, the current IB through the second coil 54 equals zero, and thecurrent IC through the third coil 56 equals −I.

If we effectuate a phase change between the phases A and B (coils 52,54), at the end of the phase change we should have IA=0 and IB=I. At aninstant t2, beginning a second transition phase I2 from the seconddriving phase D2 to a third driving phase D3, the control logic 74drives phase A (coil 52) in PWM, drives phase B (coil 54) with a dutycycle equal to 100%, and leaves phase C (coil 56) coupled to ground.Specifically, during the second transition phase I2 the control logic 74applies the first and second PWM control signals to the first high- andlow-side transistors 60, 62, respectively; a constant high controlsignal to the second high-side transistor 66; a constant low signal tothe second low-side transistor 68; a constant low control signal to thethird high-side transistor 70 and a constant high control signal to thethird low-side transistor 72.

By applying the PWM control signals to the first high- and low-sidetransistors 60, 62, the current IA through the first coil 52 will tendto increase while the first high-side transistor 60 is turned ON by thefirst PWM control signal and will tend to decrease when the firstlow-side transistor 62 is turned ON by the second PWM control signal.The time of total discharge of the first coil 52 will be thereforelonger than it would be if the same coil 52 had been immediately put intristate as in the traditional systems.

The current IB through the second coil 54 will grow with two differenttime constants according to whether the first high-side transistor 60 orthe first low-side transistor 62 is turned ON by their respective PWMcontrol signals during the second transition phase I2. When the firsthigh-side transistor 60 is turned ON, the voltage at the center tap 58is higher than when the first low-side transistor 62 is turned ON. As aresult, the time constant of the current IB through the second coil 54is lower when the first high-side transistor 60 is turned ON and higherwhen the first low-side transistor 62 is turned ON. Overall what happensis a slowing of the discharge of the first coil 52 which will be forcedto adapt itself to the time constant of the second coil 54.

The end of the second transition period I2 can be triggered by thecomparator 74 and effected by the control logic 74 as discussed abovewith respect to the end of the first transition phase I1. During theportion of the second transition phase I2 in which the first high sidetransistor 60 is ON, the current Is through the sense resistor 64 equalsIC and when the first low-side transistor 62 is ON, the current Isequals IC−IA, which equals IB. In fact, when the first low sidetransistor 62 is ON, the current through the first coil 52 recirculatesthrough the first low side transistor 62. Therefore the current IA is acurrent in the direction indicated by Iaric in FIG. 3, and thus is acurrent that is subtracted from IC to obtain the sense resistor currentIs.

In response to determining that the voltage across the sense resistor 64reaches the voltage reference Vref during the portion of transitionphase I2 in which first high-side transistor 60 is ON, the comparator 78sends a signal to the control logic 74 which causes the control logic toend the second transition phase I2 and begin the third driving phase D3.The control logic 74 continues to alternately drive current through twocoils at a time during the driving phases as shown in FIG. 4. Moreover,during each transition phases between successive driving phases, thecontrol logic 74 drives in PWM whichever coil will be kept in tristateduring the next driving phase. In doing so, the control logic 74 ensuresthat the variations of the current in the two phases in commutationduring a phase change happen in a way that maintains their sumsubstantially constant. This greatly reduces the torque ripple thatoccurs during phase commutations in prior art three-phase motors, andthereby also reduces the acoustic noise accompanying the torque ripple.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

What is claimed is:
 1. A method of driving a three-phase motor havingfirst, second, and third coils, comprising: electrically connecting thefirst coil to a first voltage reference and the second coil to a secondvoltage reference while leaving the third coil floating during a firstdriving phase; electrically connecting the first coil to the firstvoltage reference and the third coil to the second voltage referencewhile leaving the second coil floating during a second driving phase;electrically connecting the second coil alternately to the first andsecond voltage references during a first transition phase thatimmediately follows the first driving phase and immediately precedes thesecond driving phase; comparing a voltage across a sense resistor duringthe first transition phase with a third voltage reference, the senseresistor coupling the coils to the second voltage reference; andadaptively adjusting a duration of the first transition phase based onthe comparing step.
 2. The method of claim 1, further comprisingelectrically connecting the first coil to the second voltage referenceduring the first driving phase such that the first coil is electricallyconnected alternately to the first and second voltage references duringthe first driving phase.
 3. The method of claim 1, further comprisingelectrically connecting the second coil to the first voltage referenceduring the first driving phase such that the second coil is electricallyconnected alternately to the first and second of voltage referencesduring the first driving phase.
 4. The method of claim 1 wherein thefirst coil is coupled to the first voltage reference by a high-sidetransistor and to the second voltage reference by a low-side transistorand the act of electrically connecting the first coil to the firstvoltage reference during the first driving phase includes driving thehigh-side transistor and the low-side transistor with a pulse widthmodulated signal such that the first coil is alternately connected tothe first and second voltage references respectively by the high-sideand low-side transistors during the first driving phase.
 5. A method ofdriving a three-phase motor having first, second, and third coils,comprising: electrically connecting the first coil to a first voltagereference and the second coil to a second voltage reference whileleaving the third coil floating during a first driving phase, whereinthe first coil is connected to the first voltage reference by ahigh-side transistor and to the second voltage reference by a low-sidetransistor and the act of electrically connecting the first coil to thefirst voltage reference during the first driving phase includes drivingthe high-side transistor and the low-side transistor with a pulse widthmodulated signal such that the first coil is alternately connected tothe first and second voltage references respectively by the high-sideand low-side transistors during the first driving phase; electricallyconnecting the first coil to the first voltage reference and the thirdcoil to the second voltage reference while leaving the second coilfloating during a second driving phase; electrically connecting thesecond coil alternately to the first and second voltage referencesduring a transition phase that immediately follows the first drivingphase and immediately precedes the second driving phase; and drivingexclusively the high-side transistor during the transition phase andthen re-driving the high-side and low-side transistors with the pulsewidth modulated signal during the second driving phase.
 6. The method ofclaim 5, further comprising electrically connecting the third coil tothe second voltage reference during the transition phase.
 7. The methodof claim 1 wherein the second coil is coupled to the first voltagereference by a high-side transistor and to the second voltage referenceby a low-side transistor and the act of electrically connecting thesecond coil alternately to the first and second voltage referencesduring the transition phase includes driving the high-side and low-sidetransistors with a pulse width modulated signal such that the secondcoil is alternately connected to the first and second voltage referencesrespectively by the high-side and low-side transistors during the firsttransition phase.
 8. The method of claim 1 wherein a second transitionphase immediately follows the second driving phase, the method furthercomprising comparing a voltage across the sense resistor during thesecond transition phase with the third voltage reference and adaptivelyadjusting a duration of the second transition phase based on thecomparing step during the second transition phase such that the durationof the second transition phase differs from the duration of the firsttransition phase.
 9. A method of driving a three-phase motor with threecoils respectively connected to a first voltage reference by threeassociated high-side transistors and to a second voltage reference bythree associated low-side transistors, the method comprising: activatingthe transistors according to a predetermined sequence of successivedriving phases, wherein during each driving phase the high-sidetransistor associated with a first one of the three coils is activated,the low-side transistor associated with a second one of the three coilsis activated, and neither of the transistors associated with a third oneof the three coils is activated such that current flows through thefirst two coils with the third coil left floating, wherein during eachsuccessive driving stage a difference one of the three coils is leftfloating; applying a pulse width modulated first control signal tosuccessive ones of the high-side transistors during successivetransition phases between successive driving phases, for each transitionphase the high-side transistor to which the first control signal isapplied is whichever high-side transistor is associated with the coilthat will be left floating in the next driving phase; and during eachtransition phase, applying a constant activation signal to the low-sidetransistor that will be activated in the next driving stage.
 10. Themethod of claim 9, further comprising applying a pulse width modulatedsecond control signal, opposite to the first control signal, during eachtransition phase, to the low-side transistor associated with the coilthat will be left floating in the next driving phase such that the coilthat will be left floating in the next driving phase is alternatelycoupled to the first and second voltage references during the transitionphase.
 11. The method of claim 10 wherein the activating step includes,for each driving phase, driving the high-side and low-side transistorsassociated with the first coil for the driving stage with the second andfirst control signals, respectively, such that the first coil iselectrically connected alternately to the first and second voltagereferences during the driving phase.
 12. The method of claim 11, furthercomprising, for each transition phase, applying a constant activationsignal to the high-side transistor to which the second control signalwill be applied in the next driving phase.
 13. The method of claim 9,further comprising, for each transition phase, applying a constantactivation signal to the high-side transistor that will be activated inthe next driving phase.
 14. The method of claim 9 wherein each of thecoils is coupled to the second voltage reference via a sense resistor,the method further comprising, during each transition phase, comparing avoltage across the sense resistor with a third voltage reference andswitching from the transition phase to the next driving phase if thevoltage across the sense resistor is greater than the third voltagereference.
 15. A three-phase motor driver, comprising: first and secondvoltage references; first, second, and third motor coils coupled to eachother; first and second transistors coupling the first motor coil to thefirst and second voltage references, respectively; third and fourthtransistors coupling the second motor coil to the first and secondvoltage references, respectively; fifth and sixth transistors couplingthe third motor coil to the first and second voltage references,respectively; means for activating the first and fourth transistors anddeactivating the fifth and sixth transistors during a first drivingphase to electrically connect the first coil to a first voltagereference and the second coil to a second voltage reference whileleaving the third coil floating; and for activating the first and sixthtransistors and deactivating the third and fourth transistors during asecond driving phase to electrically connect the first coil to the firstvoltage reference and the third coil to the second voltage referencewhile leaving the second coil floating during a second driving phase;and alternately activating the third and fourth transistors during atransition phase that immediately follows the first driving phase andimmediately precedes the second driving phase to electrically connectthe second coil alternately to the first and second voltage references;a sense resistor coupling the second, fourth, and sixth transistors tothe second voltage reference; and a comparator having a first inputcoupled to a third voltage reference, a second input coupled to thesense resistor, and an output, the comparator being structured tocompare the third voltage reference with a voltage across the senseresistor as measured at the second input and produce at the output acontrol signal if the voltage across the sense resistor is greater thanthe third voltage reference, wherein the means for activating is coupledto the comparator output and, in response to the control signal ends thetransition phase and begins the second driving phase.
 16. The motordriver of claim 15 wherein the activating means includes means foralternatively activating the first and second transistors during thefirst driving phase to electrically connect the first coil alternatelyto the first and second voltage references during the first drivingphase.
 17. The motor driver of claim 15 wherein the activating meansincludes means for alternatively activating the third and fourthtransistors during the first driving phase such that the second coil iselectrically connected alternately to the first and second of voltagereferences during the first driving phase.
 18. The motor driver of claim15 wherein the activating means includes means for activating the firsttransistor and deactivating the second transistor during the transitionphase such that the first coil is electrically connected to the firstvoltage reference during the entire transition phase.
 19. The motordriver of claim 15 wherein the activating means includes means foractivating the sixth transistor and deactivating the fifth transistorduring the transition phase such that the third coil is electricallyconnected to the second voltage reference during the entire transitionphase.